Add a "document" style to PHUIRemarkupPreviewPanel and use it in Legalpad and Phriction

Summary: Ref T3671. Depends on D6674. Continues work in D6673, D6674 and extends it into Legalpad and Phriction. Then deletes a bunch of dead code.

Test Plan: Edited documents in Legalpad and Phriction, verified I got reasonable looking previews.

Reviewers: btrahan, Firehed

Reviewed By: btrahan

CC: aran, chad

Maniphest Tasks: T3671

Differential Revision: https://secure.phabricator.com/D6675